One of major problems in the art of EDM is detecting electrical or physical conditions in the machining gap. Thus, according to particular gap conditions existent or encountered in the EDM process, it is necessary, for example, to modify the application of discharge-producing electrical pulses to the machining gap, to control the rate of flushing the gap with the machining fluid and to regulate the conductivity of the machining medium. Further, once the machining gap suffers a contamination with machining chips, tar and other products which tends to bring about arcing, it is imperative that the gap-cleaning action be effectuated or intensified by means of vibration or retraction of the tool electrode relative to the workpiece. The follow-up feed or servo-displacement of the electrode must also be smoothly effected as machining proceeds and yet in response to change in the gap conditions.
There have in the past been proposed a number of gap-detecting methods, which may be classified into two groups. The first is to detect the gap voltage, current and/or other gap variable on an average basis and the second is to sense such variables on a per pulse basis. The averaging measurement is obviously less reliable because of its inability of instantaneous response and is therefore not adequate for prompt corrective action. On the other hand, the per pulse measurement may permit immediate countermeasures and can accordingly afford an enhanced machining efficiency. The problem is, however, that machining conditions themselves may affect parameters of individual machining pulses (i.e. peak current and on time, etc) which must be set at optimum values according to particular machining purposes (i.e., for obtaining a desired relationship of surface roughness, overcut, relative electrode wear, etc.). Hence the attempt to judge the machining conditions by sensing variables of machining pulses themselves leads most often to false results and a truly accurate determination of the gap conditions is not obtainable without adequately incorporating a change in parameters of machining pulses into sensing signals.
In a further attempt to detect gap conditions, there has also been introduced in the art pilot pulses or discharges which are used auxiliary to machining discharges. In the pilot-pulse methods which have been contemplated heretofore, a pilot signal is applied at a frontal or leading portion of each individual machining pulse to explore a pre-discharge gap condition for determining whether the gap would be in an adequate state so that the machining pulse may be triggered or for other control purposes. In these methods, the pilot "pilot pulse" is more or less integrated with the subsequent "machining" portion of each individual power pulse so that there may also be an adverse influence therefrom on each machining pulse with a set of prefixed parameters. Consequently the disadvantage mentioned earlier remain unresolved.